The present invention relates to an active matrix structure for a liquid crystal display which is employed in a flat display, for instance.
At present, display devices of the type employing liquid crystal point to a TV, a graphic display, etc. and are being developed rapidly for practical use. Above all, a liquid crystal display device of an active matrix structure having a switching element connected to each pixel is free from crosstalk and excellent in contrast, and hence is now being put into practical use as the most promising high picture quality flat display. Such a liquid crystal display device having the active matrix structure usually employs a thin film transistor as the switching element for each pixel.
FIG. 1 is a diagram showing an equivalent circuit of each pixel in a conventional liquid crystal display device having the active matrix structure. Reference numeral 11 indicates a data line, 13 a gate line, 20 a thin film transistor (TFT), and 24S the source of the thin film transistor 20, the source 24S being connected to the data line 11. Reference numeral 24D denotes the drain of the thin film transistor 20 and 13G the gate of the thin film transistor 20, the gate 13G being connected to the gate bus 13. Reference numeral 14 represents a transparent pixel electrode connected to the drain 24D, 15 a transparent common electrode formed on one of two opposed base plates (not shown) with a liquid crystal layer sandwiched therebetween, and 16 a pixel capacitance formed by the transparent pixel electrode 14 and the transparent common electrode 15.
In the liquid crystal display device of the active matrix structure, when the thin film transistor 20 is turned ON by applying a row select signal to the gate line 13, the pixel capacitance 16 is charged by a drive voltage which is applied to the data line 11 corresponding to an image signal, and the potential of the pixel capacitance 16 relative to the common electrode 15 is controlled. In other words, a voltage corresponding to the image signal is written into the pixel capacitance 16. Then, when the thin film transistor 20 is turned OFF, the voltage written in the pixel capacitance 16 is retained and stored. This mechanism is the same as that of a semiconductor memory DRAM. That is to say, the DRAM uses each capacitor as a storage element for writing therein and reading thereout information, whereas the liquid crystal display device uses the voltage of the pixel capacitance 16 to effect a molecular orientation of the liquid crystal forming a dielectric of the pixel capacitance 16 and the retention of the molecular orientation. As a result of this, the quantity of light passing through the pixel capacitance 16 is controlled, by which the function of a display element is performed.
In practice, however, the voltage written in the pixel capacitance 16 decreases owing to various leakage currents. For example, amorphous silicon used for the thin film transistor 20 is so high in photoconductivity as to be employed for a photosensor, and when the thin film transistor 20 is exposed to light, a leakage current between the source 24S and the drain 24D materially increases. On the other hand, since the conductivity of the liquid crystal itself is greatly temperature-dependent, a temperature rise significantly impairs its insulation, causing a self-discharge of the liquid crystal capacitance 16. At the same time, the leakage current of the thin film transistor 20 also increases with the temperature rise. Thus, the voltage of the pixel capacitance 16 decreases owing to such various leakage currents and the retention of the orientation of the liquid crystal becomes unstable accordingly--this is perceived as a decrease in the contrast of a display or a flicker, incurring degradation of the display quality.
To avoid this, it is a common practice to take such a measure as shown in FIG. 2. In FIG. 2, reference numeral 18 denotes a light blocking layer for blocking light incident to the thin film transistor 20. The light blocking layer 18 is formed of an opaque metal. When the thin film 20 is of an inversely staggered structure, the light blocking layer 18 is provided on the liquid crystal side, whereas when the thin film transistor 20 is of a staggered structure, the light blocking layer 18 is provided on the base plate with an insulating film sandwiched therebetween. Reference numeral 17 indicates a storage capacitance electrode disposed opposite the transparent pixel electrode 14. The storage capacitance electrode 17 is connected to a storage capacitance line dedicated thereto or to the gate line 13 of the preceding stage. Reference numeral 19 denotes a signal storage capacitance formed by the transparent pixel electrode 14 and the storage capacitance electrode 17. A dielectric of the signal storage capacitance 19 is formed by a silicon oxide film, silicon nitride film, or similar stable insulating film which has an excellent insulating property.
The liquid crystal display device of such an active matrix structure includes the light blocking layer 18 as mentioned above, and hence affords substantial reduction of the leakage current which results from the incidence of light to the thin film transistor 20. Furthermore, since the signal storage capacitance 19 is provided in parallel to the pixel capacitance 16, the amount of charge stored in the total capacitance can be greatly increased, and consequently, even if the self-discharge of the pixel capacitance 16 and the leakage current of the thin film transistor 20 increase owing to a temperature rise or the like, the voltage variation of the pixel capacitance 16 can be held within a given limit. Thus, the stability of orientation of the liquid crystal increases and a decrease in the contrast of a display can be avoided.
Since such a liquid crystal display device includes the light blocking layer 18 and the signal storage capacitance 19, however, the number of its manufacturing steps increases and the manufacturing cost rises accordingly. For instance, the formation of the light blocking layer 18 calls for the steps of depositing metal, patterning it and depositing an insulating film. When the storage capacitance electrode 17 is connected to the gate line 13 of the preceding stage, there is no need of providing a dedicated storage capacitance line, and consequently, the number of manufacturing steps does not increase, but the load on the gate line 13 increases so much that it is difficult to apply the liquid crystal display device to a large display, and at the same time, the necessity of sequential scanning imposes severe limitations on the drive system used. On the other hand, when the storage capacitance electrode 17 is connected to the storage capacitance line, the load on the gate line 13 is so small that the liquid crystal display device can be applied to a large display, but the formation of the storage capacitance line increases the number of manufacturing steps and hence raises the manufacturing costs.